In communication devices designed to operate in different modes, such as multi-band capable cellular devices, multiple amplifiers are typically used corresponding to each mode of operation. Each amplifier may, for example, amplify receive signals corresponding to an associated cellular technology, such as Global System for Mobile Communication (GSM), Code Division Multiple Access (CDMA), Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMax), Wireless Local Area Network (WLAN) and Bluetooth or other Personal Area Networks (PAN). This is because each amplifier must be designed to maximize associated performance characteristics, such as very low 1/f noise, increased bandwidth, or ability to operate at higher frequencies, for the corresponding cellular technology.
FIG. 1 is a high level block diagram of a conventional device 100 with plural amplifiers 104 and 105, each for amplifying the desired incoming signal when operating in multi-mode. In the particular example, device 100 is capable of processing receive signals for both GSM and LTE cellular technologies. The electromagnetic waves containing the received signals are absorbed by antenna 101 and selectably routed by receiver logic 106 to amplifier 104 (associated with the GSM mode of operation) or to amplifier 105 (associated with the LTE mode of operation) by appropriately enabling and disabling controls switches 102 and 103. When operating in GSM mode, switch 102 is closed and switch 103 is open, thus allowing the received signal to flow to GSM amplifier 104 and preventing the received signal from flowing to LTE amplifier 105. Amplifier 104 is configured to provide low noise performance to meet the low noise requirements of the GSM cellular technology. Amplifier 104 may achieve this low noise performance by utilizing a large input transistor pair. Larger transistors exhibit less 1/f noise because larger transistors have larger gate capacitances, which smoothes the fluctuations in channel charge. Thus, the larger the transistor the lower the resulting 1/f noise. The mean-square 1/f drain noise current can be expressed as follows:
                              i                      -            2                          =                              (                          K              ⁢                              /                            ⁢              f                        )                    ⁢                      (                                                            g                  m                  2                                /                W                            ⁢                                                          ⁢              L              ⁢                                                          ⁢                              C                ox                2                                      )                    ×          B          ⁢                                          ⁢          W                                    Eq        .                                  ⁢                  (          1          )                                    where, W is the gate width, L is the gate length, Cox is the transistor gate capacitance, gm is the transistor transconductance, f is the operating frequency, K is an empirical constant and BW is the noise bandwidth of the transistor. Thus, an increase in transistor gate area results in a decrease in transistor 1/f noise.        
When operating in LTE mode, switch 103 is closed and switch 102 is open, thus allowing the received signal to flow to amplifier 105 and preventing the received signal from flowing to GSM amplifier 104. The LTE amplifier 105 is configured to provide high frequency performance to meet the frequency requirements of the LTE cellular technology. LTE amplifier 105 may achieve this high frequency performance by utilizing a small input transistor pair. Smaller transistors exhibit higher operating frequencies because smaller transistors have smaller gate capacitances, which reduce the time necessary to charge and discharge the transistor. Transistor unity gain frequency can be expressed as follows:WT=gm/(Cgs+Cgd)   Eq. (2)                where, gm is the transconductance of the transistor, Cgs is the gate-to-source capacitance and the Cgd is the gate-to-drain capacitance.        
FIG. 2 is a low level circuit diagram of the device shown in FIG. 1. Amplifier 104 is comprised of large transistors 203 and 204 coupled in a common source configuration to current source 205. The drain of large transistor 203 is coupled to a first terminal of resistor 206. The drain of large transistor 204 is coupled to a first transistor of resistor 207. Resistors 206, 207 serve as active loads. A second terminal of resistor 206 and resistor 207 is coupled to power supply VDD.
LTE amplifier 105 comprises small transistors 208 and 209 coupled in a common source configuration to current source 210. The drain of small transistor 208 is coupled to a first terminal of resistor 211. The drain of small transistor 209 is coupled to a first terminal of resistor 212. A second terminal of resistor 211 and 212 is coupled to power supply VDD.
According to the required performance characteristics described above, device 100 is capable of selecting the desired amplifier by enabling and disabling selected ones of switches 213-220. When for example operating in GSM mode, switches 214, 216, 219 and 220 are closed and switches 213, 215, 216 and 218 are opened. This switching configuration grounds the gate terminals of transistors 208 and 209 of amplifier 105 and diverts the input signal Vin + and Vin − to the gate terminals of transistors 203 and 204 of amplifier 104; thus enabling GSM mode operation and preventing amplifier 105 from becoming operational.
By contrast, when operating in LTE mode, switches 214, 216, 219 and 220 are opened and switches 213, 215, 216 and 218 are closed. This switching configuration grounds the gate terminals of transistors 203 and 204 of amplifier 104 and diverts the input signal Vin + and Vin − to the gate terminals of transistors 208 and 209 of amplifier 105; thus enabling LTE mode of operation and preventing amplifier 104 from becoming operational.
Thus, conventional devices use plural amplifiers to achieve desired performance characteristics for each mode of operation by utilizing a separate amplifier circuit for each mode of operation.